• Nathan Fox

Right for the Wrong Reasons

Test 15 - Section 3 - Question 18

Logical Reasoning

Difficulty: 5


Demon user Erica C asks,

"Can you explain? I got the answer correct, but my decision was between the correct answer and C. I'm not sure how to explain that C is wrong besides A is more definitive and sounds more like a premise whereas C leaves things uncertain."

Thanks Erica! I love getting questions like this. Students often get questions right for the wrong, or incomplete, reasons. By striving for full clarity, Erica is saying "I don't just want to fall backward into the correct answer; I want to know the answer so I can smash this test right in the face." Hell yeah. Let's do this.

Sufficient Assumption is one of the most smashable question types. We're asked to prove the conclusion. We're not just strengthening. We're winning—with certainty. There's usually only one way to get there. There are no second-best answers. We're not going to do this one passively, jumping to the answer choices too quickly and searching for the "best" answer. Instead, we're going to do this actively. We're going to predict the answer. It's our turn to tell them what the answer is—not the other way around.

The argument is bogus because it assumes that the randomness shown in one study of bacteria must hold for all genetic mutation everywhere. Not just in these bacteria, but in all bacteria. And not just in all bacteria, but in all creatures big and small. Even polar bears. That's an enormous leap.

If we're going to prove this argument, we have to cover the entire gap; the correct answer must prove the argument's conclusion, given its facts. So we'll predict an answer that links the evidence to the conclusion. The answer I'm looking for is "if one study shows something, that thing is always true everywhere."

A) This answer has the same effect as my prediction even though it uses different words. The facts described one instance in which genetic mutation was random. If A is true, then all genetic mutation everywhere has to be random. This answer, if true, proves the argument's conclusion. So this is the correct answer.

B) This doesn't even strengthen the argument. Who cares if the bacteria were of a common type? How does that prove that the same results would hold for uncommon types? Let alone polar bears?

C) This answer is a non-starter because the facts didn't show that all genetic mutation in bacteria is random. The facts were only about one study of bacteria. We don't even know if those results were ever replicated on the exact same bacteria. This answer doesn't prove anything. So it's conclusively wrong.

D) This might weaken the argument. It certainly doesn't prove it.

E) How does this prove that all genetic mutation everywhere is random? It doesn't. At all. 

The correct answer is A because it's the only one that proves the conclusion. Keep your standards high on Sufficient Assumption questions. Always predict an answer. Then, when looking at the answer choices, expect the correct answer to make it <click>. The conclusion is proven, or it's not. There is no second-best.

Get more of these explanations from the LSAT Demon


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